Environmental Engineering Reference
In-Depth Information
Figure 6.19
Different methods to incorporate reactive elements [39].
of the elements/oxides or ion implantation has the following beneficial effects
on the oxidation behavior of the alloys:
1.
The growth rate of Cr
2
O
3
scales is undoubtedly reduced and that of the Al
2
O
3
scales may be reduced (controversial).
2.
Selective oxidation of the protective scale-forming element, i.e., Al or Cr,
is enhanced, particularly during the early stages of oxidation. That is, contin-
uous protective scales of Cr
2
O
3
or Al
2
O
3
are achieved at lower Cr or Al
contents of the alloy compared with that in addition-free alloys.
3.
The transport mechanism of Cr
2
O
3
growth is changed from predominantly
outward cation (Cr
3
) transport to oxygen ingress. There is no evidence to
suggest a similar change with Al
2
O
3
-forming alloys, for which the oxygen
transport, apparently via grain boundary short-circuit paths, appears to be
